Or male animals... Up till now, I've always used females. Many of my colleagues who work with large animals find females easier than males. Believe me, I will publish showing that in the systems in which I work, inter-individual variation swamps inter-sex variation to the point where the latter is not significant.

We use everything we can get regardless of sex. In complicated genetic crosses where we only get 1/16 animals with two floxed alleles, a Cre transgene, and a reporter, we aren't going to be picky about whether it happens to be a male or female.

If the results are negative (i.e. no sex differences), they should reduce their n's and just go half and half in all experimental groups from then on. That is the point of the initiative, that you just use both.